WO2017156320A1

WO2017156320A1 - A new radiation detection material and enhanced radiation portal monitor

Info

Publication number: WO2017156320A1
Application number: PCT/US2017/021656
Authority: WO
Inventors: James K. Walker; Youngwook Noh; Richard T. FARLEY
Original assignee: Nanoptics, Incorporated
Priority date: 2016-03-09
Filing date: 2017-03-09
Publication date: 2017-09-14

Abstract

Embodiments relate to a method and apparatus for detecting radioactive and nuclear materials. Embodiments are directed to a method and apparatus for detecting neutrons and gamma rays. Embodiments relate to a low cost, scintillating material composition using an organo-metal scintillating material and Gd that can form a large area detector of neutrons and gamma rays with good discrimination between the two forms of radiation. Embodiments of a detector incorporating such a scintillating material can act as a passive detector to detect RDD and lightly to moderately shielded SNM or a detector used with active interrogation to detect RDD and to detect lightly, moderately, and heavily shielded nuclear material. Embodiments are used for detecting radiological and nuclear threats where a single material composition is used for both the neutron and gamma ray detection in the detector system. The nuisance alarm rate during detection of RDD is very low and allows a high flow of commerce. Operation of the detection system is unaffected by severe changes of the environment and assures both short term and long term stability and low life cycle costs.

Description

A NEW RADIATION DETECTION MATERIAL AND ENHANCED RADIATION

PORTAL MONITOR

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application Serial No.

62/305,799, filed March 9, 2016, which is hereby incorporated by reference herein in its entirety, including any figures, tables, or drawings.

BACKGROUND OF INVENTION

The background of detection of gamma rays and fast/thermal neutrons has been well described in the past and, in particular, is well described in U.S. Patent No. 8,963,094. Physical shipment of materials, including the shipment of mail, merchandise, raw materials, and other goods, is an integral part of any economy. Typically, the materials are shipped in a type of shipping container or cargo box. Such containments or boxes include semi-trailers, large trucks, and rail cars, as well as inter-modal containers that can be carried on container ships or cargo planes. However such shipping or cargo containers can be used for illegal transportation of contraband, such as nuclear and radioactive materials. Detection of these threats requires a rapid, safe, and accurate inspection system for determining the presence of hidden nuclear materials, especially at state and national borders, along with transit points, such as airports and shipping ports. Currently, both passive and active detection techniques are employed for the detection of concealed nuclear and radioactive materials.

Passive detection techniques for the detection of concealed nuclear and radioactive materials are based on the principle that nuclear and radiological threats emit gamma rays (gamma radiation), and in some cases, neutrons (neutron radiation) that can be detected. Although passive systems can be easily deployed, some passive systems suffer from a number of drawbacks, including high rates of nuisance and false positives, and misdetections caused by unavoidable factors, such as depression of the natural background by the vehicle being scanned and the contents of the vehicle being scanned, variation in natural background spectrum due to benign cargo such as clay tiles and fertilizers, and the presence of radio therapeutic isotopes in the cargo having gamma lines near threat lines. Further, many gamma sources are self-shielded and/or can readily be externally shielded, which makes them difficult to detect, since the radiation given off is absorbed in the shielding. Further, in general, gamma detectors make poor neutron detectors, and good neutron detectors tend to be poor gamma detectors.

Active detection techniques for the detection of concealed nuclear and radioactive materials typically employ beams of uncharged particles, such as neutrons and photons (e.g., gamma rays) to irradiate suspicious containers. Uncharged particles have the potential to penetrate relatively large, dense objects to identify particular elements of interest; thus, some detection devices utilize the absorption and/or scattering patterns of neutrons or photons as they interact with certain elements present in the object being inspected. Examples of such devices can be found in U.S. Patent Nos. 5,006,299 and 5, 114,662, which utilize thermal neutron analysis (TNA) techniques for scanning luggage for explosives, and in Patent No. 5,076,993, which describes a contraband detection system based on pulsed fast neutron analysis (PFNA). These patents are incorporated herein by reference in their entirety.

Active detection techniques for the detection of concealed nuclear and radioactive materials, such as Differential Die away Analysis (DDA) and measurements of delayed gamma- ray and neutrons following either neutron-induced or photon-induced, fission, can be used to detect the presence of fissile materials. The radiation is measured with neutron and gamma-ray detectors, and systems, preferentially insensitive to each other's radiation. Detection of delayed neutrons is an unequivocal method to detect fissile materials, even in the presence of shielding mechanism(s) to hide the nuclear materials. Because the number of delayed neutrons is frequently two orders of magnitude lower than the number of delayed gamma rays, efficient, large area detectors are required, with good gamma discrimination, to achieve the best sensitivity in neutron detection.

Each of the detector systems described above has drawbacks. In particular, these devices generally utilize accelerators that produce high energy neutrons with a broad spectrum of energies. The absorption/scattering of neutrons travelling at specific energies is difficult to detect given the large number of neutrons that pass through the object without interaction. Thus, the "fingerprint" generated from these devices is extremely small, difficult to analyze, which often leads to significant numbers of false positive or false negative test results. In addition, known prior art detection systems have limitations in their design and the methods they utilize that require them to have high radiation doses, which often poses a risk to the personnel involved in inspection, as well as to the environment.

While the use of both passive and active detection techniques is desirable, what is needed is a neutron and gamma-ray based detection system and method that is cost-effective, and uses a large area detector that is preferably fabricated from readily available materials.

The most commonly used neutron detector is a He-3 gas proportional chamber. However, He-3 is a relatively scarce material, which does not occur naturally. This makes the availability and future supply of such detectors somewhat uncertain. As a result, detectors employing other detector materials, such as detectors containing boron, have been used in tubes and plates to achieve substantially similar neutron detection efficiencies as the HE-3 gas proportional chamber (see, e.g., U.S. Patent No. 8,963,094).

The most common globally deployed gamma ray detector for use with passive radioactive material detection employs a plastic scintillator (polyvinyl toluene PVT) with a PMT (photo multiplier tube). Several designs of detectors utilizing a plastic (PVT) with a PMT have been developed. The introduction of on-line software to deconvolve the measured pulse height spectra from plastic scintillators, by Burt and Ramsden; Nuclear Science Symposium Conference Record, IEEE, pages 1186-1190, 2008, made a major contribution to improving the energy resolution of gamma ray detection for gamma ray energies less than 3 MeV. As a result, isotope identification was improved and nuisance and false alarm signals were substantially lowered. Symetrica is a UK company that has developed the deconvolution technique to good effect for spectroscopic radiation detection. Symetrica has shown how to improve the existing PVT plastic scintillator technology for gamma ray detection in both their hardware and deconvolution software. Symetrica has shown how to achieve identification and rejection of signals from naturally occurring radioactive material (NORM), thereby reducing false alarm rates. While this improvement in gamma ray detection is advantageous, the intrinsic sensitivity of such systems to RDD and SNM has been reduced by having to use thinner (4 cm) sheets of PVT to achieve the necessary resolution on the gamma ray Compton spectra. As a result, the intrinsic detection sensitivity of 1 to 2 MeV gamma rays is reduced to about 25%. In addition, it has been discovered that high humidity and changing temperatures can substantially impact the efficacy of these detectors. Their gamma energy resolution is impacted and life cycle costs are increased.

An example of a neutron and gamma based detector system is disclosed in U.S. Patent No. 8,963,094, which discloses a detector composed of multiple cells, which are stacked together. One type of cell is composed of gas proportional detectors sensitive to neutrons. Another type of cell is composed of plastic scintillator sheets to detect gamma-rays. These sheets also act as moderators of fast neutrons. The cells can be employed in various configurations within different stacking arrangements. In addition, B-10, Li-6, Cd, and gadolinium layers may be used as deposited material on one or another of the cells to capture thermal neutrons.

Such neutron and gamma detection systems are able to detect delayed neutrons and gamma rays from nuclear material that has received active interrogation. In this way, such detector systems are able to detect "moderately shielded" nuclear materials in a container with good efficiency. However, some containers may have shielding such that the nuclear materials in the container may be characterized as being "well shielded". In certain situations, the shielding has been designed to evade detection of the nuclear material by current detection systems. Such shielding is typically designed to have enough high Z material to heavily absorb the usual 0.1 to 3 MeV delayed gamma rays, and enough hydrogenous material to moderate and absorb the delayed neutrons in cadmium or other absorbing materials. Specifically, the shielding may be designed to substantially reduce the delayed signal of neutrons and gamma rays from nuclear material to the point that detection is evaded, or at least very difficult.

It is reasonable to expect that terrorists having the necessary technical knowledge and ability to illegally import nuclear material, are likely to also have the knowledge to make the material more than "moderately shielded". However, detectors presently deployed and detectors presently planned for field deployment, for detection of nuclear materials, while being capable of detecting "moderately shielded" nuclear materials in containers, are probably not sufficiently sensitive to detect "well shielded" nuclear material. This potential limitation with respect to detecting illegally imported "well-shielded" nuclear material, and, in particular, highly enriched uranium (HEU), represents a serious potential existential threat to the security of the USA and other advanced countries. In the last five years there has been intense effort in both the US and Europe to develop improved detection of Radioactive Dispersal Devices (RDDs) and Special Nuclear Material (SNM) at border crossings. A driving requirement for new radiation portal monitors (RPMs) to be deployed in Europe or the USA is a neutron and gamma ray based detection system that can be made in large area, at low cost, and has adequate spectral resolution for gamma ray detection to dramatically reduce NORM backgrounds. In passive mode, these portals should be able to detect radioactive materials that could be used for radioactive dispersal devices (RDDs), or dirty bombs. Also, these portals should be able to detect lightly shielded SNM. In active mode, these portals should be able to detect moderately shielded nuclear materials. The general objectives of the planned new portals are described in Scintilla: A new international platform for the development, evaluation and benchmarking of technologies to detect radioactive and nuclear material (2013), published in Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2013 3rd International Conference, 23-27 June 2013. The Final Report Summary (hereinafter "Scintilla Summary") - SCINTILLA (Scintillation Detectors and New Technologies for Nuclear Security) was issued on December 31, 2014, which is incorporated herein by reference in its entirety.

The development of alternatives to Helium-3 based technologies for neutron detection has been pursued by SCINTILLA and reviewed by a series of workshops organized by NNSA/DOE (Department of Energy) and EURATOM. There appear to be several techniques that meet the necessary specifications, including good enough gamma rays discrimination, which were established by the Helium-3 detectors in RPMs. One of these techniques, based on LiF/ZnS, has been tested by Symetrica with full scale RPM detectors and operated successfully at the Rotterdam port facility. The intrinsic detection sensitivity of fast neutron detection was shown to be about 8%, comparable to the original Helium-3 technology employed in RPMs. A characterization can be made that the Spectroscopy-RPM (S-RPM) from Symetrica as having a gamma /neutron intrinsic detection efficiency of (25 % / 8 %) and very good suppression of nuisance alarms.

Other techniques, using Boron based proportional tubes, have been shown to have similar neutron detection characteristics. These low intrinsic detection efficiencies of the prior methods leave room for improvement. Accordingly, there is a need for a substantial improvement of intrinsic detection efficiencies and, therefore, increased sensitivity for detection of RDD and SNM when transported illegally. If cost could be controlled, it would be highly desirable to go beyond these generally accepted American and European Standards for portals. While it is important to have spectroscopic gamma ray detection with reduced false, or nuisance, alarms, there is a continuing need for improved intrinsic detection efficiency of gamma rays and neutrons, in order to enhance the detection sensitivity for the detection of shielded RDD and SNM. Embodiments of the subject of this invention provide one or more cost effective material compositions, one or more detectors, and one or more methods of achieving one or more of these objectives.

BRIEF SUMMARY

Embodiments of the invention relate to an apparatus for detecting radioactive and nuclear materials, specific embodiments are directed to a method and apparatus for detecting neutrons and gamma rays. Embodiments of the invention relate to a low cost, scintillating material composition that can form a large area detector of neutrons and gamma rays with good discrimination between the two forms of radiation. Embodiments of the invention relate to a low cost, organo-metal scintillating gel-type material composition that can form a large area detector of neutrons and gamma rays with good discrimination between the two forms of radiation. Embodiments of a detector incorporating such a scintillating material can act as a passive detector or a detector with active interrogation. Specific embodiments relate to a neutron and gamma-ray based detection system and method that is cost effective, and fabricated using an organo-metal scintillating material and Gd.

An embodiment of a portal, or detector system, in accordance with the subject invention can be referred to as a Sensitive Spectroscopic Radiation Portal Monitor (SS-RPM).

Embodiments of the subject detector are operated in passive mode, and are designed to operate in passive mode with increased sensitivity to RDD and SNM. Embodiments of the subject SS-RPM invention are designed to have at least a 3-fold, at least a 4-fold, and/or at least a 5-fold increased sensitivity to these threat sources when operated in passive mode operation, compared with the SCINTILLA portals (which incorporate plastic scintillators using Compton Edge data for gamma resolution). Table 1 shows a comparison of the gamma detection results from the two types of monitors: S-RPM and SS-RPM.

Table 1. Comparison of results from the Spectroscopic-Radiation Portal Monitor (S- RPM) using the Spectroscopic Plastic Scintillator Plate Detector and the response of the Sensitive Spectroscopic-Radiation Portal Monitor (SS-RPM) using an embodiment of the organo-tin gel material in Table 2.

A specific embodiment of a detector in accordance with the subject invention using the scintillating material described in Table 2 in combination with Gd e.g., Gd dissolved in the scintillating material or Gd positioned separate from the scintillating material, meets one or more of, and preferably all of, the following objectives:

1) The portal, or detector system, has greater than 2, greater than 3, greater than 4, and/or greater than 5 times better intrinsic detection efficiency for gamma ray source detection than the SCINTILLA portals, where the portal, or detector, utilizes an upgraded PVT, and utilizes deconvolution software;

2) The portal, or detector system, has greater than 2 times, and preferably greater than 3 times, better intrinsic detection efficiency for neutron detection than the SCINTILLA portals, such as those based on either boron based proportional tubes or 6LiF:ZnS(Ag) based technology; and 3) Within a fixed number of portal enclosures, and a fixed total detector area, the SS- RPM detector has larger sensitive area for both gamma ray and neutron detection. This embodiment of the subject invention achieves this objective by employing a single material, as described in Table 2 and Table 3, for detection of both types of radiation, which to a first order, will double the sensitive area compared to the S-RPM.

The sensitivity for detecting a threat source by such a portal is proportional to the intrinsic detection efficiency of the portal times the sensitivity area of the portal. As a result, in a specific embodiment, operating the detector in passive mode has at least 3, at least 4, and/or at least 5 times, and preferably 6 to 10 times, the sensitivity for detecting RDD and SNM than the present plans for portals, or monitors, discussed in the Scintilla Summary.

In the passive mode, a detector using the scintillating material described in Table 2 in combination with Gd provides substantially increased sensitivity, compared to existing deployed RPM, for detection of RDD material, and operates with a low rate of nuisance alarms due to NORMS. In addition, the detector has substantially increased sensitivity to detect Plutonium and HEU. However, even moderate shielding around concealed HEU can make the HEU difficult to detect. In the active mode of interrogation, the capability of the SS-RPM is enhanced to provide, in addition to detection of RDD, high detection sensitivity of Plutonium and HEU even when they are moderately to well shielded.

In one embodiment, when operated in passive mode, the detector provides for sensitive detection of radiological material with a low rate of nuisance, or false, alarms, as gamma rays are measured with a high photopeak to total ratio. The measured gamma ray energy resolution is about 17% / Square Root E (MeV). When the measured spectrum is subjected to on-line deconvolution, the resolution of the deconvoluted measured spectrum is improved by about a factor of three, and preferable improved by about a factor of 4, compared to the original raw spectra (measured spectrum). The resulting enhanced spectra (deconvoluted measured spectrum) contains only full-energy peaks, where all counts are "reassigned" to a spectral line.

Library-based line identification methods can be used for radionuclide identification based on the spectral line. The library of potential radioactive sources includes the 40 isotopes typically used for industrial sources, including sources that can be used for making Radionuclide Dispersal Devices (RDDs or Dirty Bombs), and natural sources (NORM). In the SS-RPM, the scintillating material is contained in a metal enclosure for each detector unit. This eliminates any effect of high or changing humidity on the detector operation. As a result, this represents a major advantage over the S-RPM detector which, as described earlier, has a NORM rejection rate sensitive to the humidity and temperature environment. Thus the SS-RPM provide a high and consistent flow of commerce.

The passive detection of threat sources to make RDD, and/or RDD devices, by the subject detector is both efficient, stable and offers high discrimination against nuisance and false positive alarms. Plutonium can be detected with the detector, with high sensitivity because of the strong emission of neutrons by plutonium. Highly Enriched Uranium (HEU) can be detected with the detector, even though HEU has relatively weak neutron emission, because the detector (neutron detector) has a high neutron detection sensitivity. However, the signature gamma ray line from HEU decay is at 186 Kev, which is at a low enough energy to be significantly absorbed by even 1 cm of stainless steel. As a result, 1) passive operation of the detector is capable of stable, sensitive detection of RDDs with acceptable discrimination against nuisance alarms from NORMs; 2) the detector can measure the presence of even small quantities of shielded Plutonium; and 3) the detector can measure the presence of HEU by compensating for the weak neutron emission of HEU with a high detection sensitivity of neutrons. However, even moderate neutron shielding will render it difficult to detect HEU with the detector.

Accordingly, the SS-RPM detector, based on the material described in Table 2 provides a distinct advantage over existing or planned passively operated RPMs, but detection of moderately shielded HEU will still be challenging with the detector.

In active interrogation mode, the subject system using the material described in Table 2 in combination with Gd is a very powerful detector of moderately shielded nuclear material and moderately shielded HEU. Active detection techniques, such as Differential Die-away Analysis (DDA), using measurements of delayed gamma-rays and neutrons following either neutron- induced or photon-induced, fission, can be used to detect the presence of moderately shielded fissile nuclear materials.

Normally, the detection of delayed neutrons from HEU, after neutron-induced or photon- induced fission, is the only viable detectable signal from moderately shielded HEU. However, it is important to recognize there may be a well-designed shielding positioned around the HEU, where the shielding is e.g., composed of a combination of hydrogenous material to reduce the energy of fast neutrons, and interspersed with cadmium to capture the thermalized neutrons. This type of shielding will reduce the ability to detect delayed neutrons after the neutron-induced or photo-induced fission, and, hence, this type of shielding will reduce the ability to detect such shielded HEU. The optimum combination of these two shielding techniques, i.e., hydrogenous material with an efficient thermal neutron absorber, minimizes the number of delayed neutrons arriving at the detector. It is expected that a need will exist to detect nuclear material shielded by this type of "well designed shielding", i.e., "heavily shielded," nuclear material, transported by determined, technically competent terrorists. There are no proposed or planned Radiation Portal Monitors (RPMs) that are designed to detect such heavily shielded SNM, and, in particular, such heavily shielded HEU.

In addition to the conventional detection techniques described above, embodiments of the invention relate to a method to detect "heavily shielded" HEU nuclear material. Embodiments of the subject detector operating in active mode, are designed to detect delayed gamma rays after fission, with energies up to 7 MeV with high intrinsic efficiency and good energy resolution. Such delayed gamma rays have energies in the range 3-7 MeV and are uniquely produced by the fission actinide isotopes produced from activated nuclear material. These energetic gamma rays are highly penetrating through the "well designed shielding of the heavily shielded" SNM material. Importantly, there are no sources of the 3 - 7 MeV gamma rays from NORMs, or industrial radioactive sources or medical radioactive sources, or any other known background radiation other than cosmic rays.

In an embodiment, scintillating material such as the scintillating material in Table 2 can be incorporated in a unit cell detector that is employed as a component in a Sensitive Spectroscopic RRM (SS-RPM), with the purpose of detecting radiological and nuclear materials (RN) with substantially increased sensitivity compared to any previous monitor.

In an embodiment, the scintillating material of this unit cell detector comprises organo and organo-metal compounds, and fluorescent dyes.

In an embodiment, the organo and organo-metallic scintillating material composition is a low viscosity liquid. In an embodiment, the organo and organo-metallic scintillating material composition is a viscous material.

In an embodiment, the organo-metallic scintillating material composition is a gel-like material which is non-pourable.

In an embodiment, the organo-metallic scintillating material composition is a rigid cross- linked material.

In an embodiment, the organo and organo-metallic scintillating material is contained within a container made from a metal sheet, where the inner surface of the metal container is highly reflective, with at least one end of the container made from glass, or other transparent or at least partially transparent material, through which the scintillation light produced during detection is transmitted and detected by photo detectors, or sensors such as, photo multiplier(s) or solid state devices.

In an embodiment, the metal container can have transverse dimensions of 4" x 4", 6" x 6", 8" x 8" or larger.

In an embodiment, the metal container can have a length of 6", 8", 10", 12", 14", 24",

36" or 48".

In an embodiment, a thin gadolinium layer, made of, for example, a solid gadolinium compound, is positioned near the container containing the organo and organo-metallic scintillating material, such as positioned to one side of the container.

In an embodiment, two metal containers containing the organo-metallic scintillating material are placed on opposite sides of the gadolinium layer.

In an embodiment, the two containers with gadolinium layer positioned between the two containers is employed as a spectroscopic gamma ray detector and simultaneously employed as a neutron detector.

In an embodiment, one or more unit cells detectors are positioned within a EMA enclosure.

In an embodiment, a unit cell detector, whose scintillation light is detected by one or a small number of photo multipliers or solid state detectors, where the material has dissolved within it a concentration of gadolinium in the range 0.05% wt./wt. up to 0.2% wt./wt. In an embodiment, a plurality of unit cell detectors used to detect the neutrons and gamma rays are placed adjacent to each other to achieve a detector area of up to 1, up to 2, between 1 and 2 and/or more than 2 square meters.

In an embodiment, the organo and organo-metallic scintillating material can be incorporated in one or more small metal unit cell detectors that can fit in a backpack.

In other embodiments of the invention, the detector units can be used to scan cargo for aircraft, used in small maritime boats, and used in automobiles or other land vehicles for monitoring large areas.

Embodiments of the subject invention are directed to a material, apparatus, and method for detecting and identifying threat related radioactive material and special nuclear material in a container wherein:

1) The detecting material is a transparent organo-metallic material composed of fluorescent compounds, organo-tin and other organo-metallic compounds and forms of linear and/or cross-linked polymer material.

2) The detecting material emits light from radiation impinging on it.

3) The detecting material is low to highly viscous and/or gel like with controllable structural strength.

4) The detecting material is contained in a metallic structure wherein the scintillation light from the material is transmitted through the scintillating material, reflected by the walls of the container and exits a glass window and detected by photoelectric devices.

5) Gadolinium is contained in the organo-metallic scintillating material or is located immediately outside the metal box.

6) The electric signals produced thereby are processed on-line to provide information on the type of radiation, gamma ray or neutron, striking the detecting material, its energy and time of arrival.

7) The information provides enough energy resolution to distinguish the sources of radiation being benign or threat related.

8) RDD sources can be detected with high sensitivity without restricting commerce.

9) S M can be detected in passive mode and also in active mode. 10) In active mode, the delayed high energy (3-7 MeV) gamma rays from actinide nuclear decays can be detected with adequate energy resolution to detect SNM with little background.

11) In active mode, delayed neutrons can be detected as a signature of SNM.

12) In active mode, the high counting rate ability of the organic scintillation process permits operation in the high backgrounds associated with intense beams used in active interrogation.

13) The scintillating material is contained in metal units such that they are not affected or deteriorated by the environment.

The aforementioned and other embodiments of the present invention shall be described in greater depth in the drawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be appreciated, as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Figure 1A shows the measured spectrum of scintillation pulse heights from gamma rays using a Cesium source of 663 KeV gamma rays incident on a 3" x 3" x 3" cup containing a specific composition of an organo and organo-metal scintillating material described in Table 2, where a clear photo peak is observed.

Figure IB shows the measured spectrum of scintillation pulse heights from gamma rays using a Cesium source of 663 KeV gamma rays incident on a 3" x 3" x 3" cup containing a specific composition of an organo and organo-metal scintillating material described in Table 2, where a clear photo peak is observed, and the deconvolved spectrum has an energy resolution of 4.3% FWHM.

Figure 2 shows the results of a Monte Carlo calculation of the average deposited energy fraction of an incident 5 MeV energy gamma ray in an 8"x8" cross-section detector as a function of detector thickness, where the scintillating material is an organo and organo-metal scintillating material having a composition containing 20% wt./wt. of tin. Figure 3 shows a schematic unit cell of the gamma ray detector illustrated with area dimensions of 8" x 8" and an 8" length, where this length is adequate to contain 90% of the energy of a 5 MeV gamma ray.

Figure 4 shows a detector array with dimensions 40" x 40" (approximately lm x lm) incorporating 25 (a 5 x 5 array) of the unit cells shown in FIG. 3.

Figure 5 shows a unit cell of a detector, where the unit cell of the detector has two metal containers 4" x 8" x 63" with organo and organo-metal scintillating material within the containers, and the two containers separated by a thin sheet of material containing gadolinium (Gd), such as a Gd layer.

Figure 6 shows a EMA enclosure ( EMA RPM enclosure) containing four unit cells as shown in FIG. 5.

Figures 7A and 7B show a detector having an array of unit cells, where each unit cell of the detector has dimensions 6" x 6" x 12" depth, and a thin sheet of material containing gadolinium (Gd), such as a Gd layer, between units.

Figure 8 shows data of the typical Pulse Shape Discrimination (PSD) between neutrons and gamma rays, using a 3" x 3" x 3" cup containing a specific composition of an organo- metallic scintillating material described in Table 2.

Figure 9 shows spectra displaying the typical Pulse Shape Discrimination (PSD) between neutrons and gamma rays, using two 8" x 8" x 4" containers in a back to back configuration of the specific composition of organo- metallic scintillating material described in Table 2, and separated by a thin Gd plate.

Figure 10 shows a schematic of the cross-sectional view of a 4 x 4 array of detector units described in Figure 7A and 7B, where an incident neutron is thermalized and captured by gadolinium and the emitted gamma rays with total energy of 8 MeV are shown entering several detector units.

DETAILED DISCLOSURE

Embodiments of the invention relate to a method and apparatus for detecting radioactive and nuclear materials. Specific embodiments are directed to a method and apparatus for detecting neutrons and gamma rays. Embodiments of the invention relate to a low cost, scintillating material composition that can form a large area detector of neutrons and gamma rays with good discrimination between the two forms of radiation. Embodiments of the invention relate to a low cost, scintillating gel-type material composition that can form a large area detector of neutrons and gamma rays with good discrimination between the two forms of radiation. Embodiments of a detector incorporating such a scintillating material can act as a passive detector or a detector used with active interrogation.

Specific embodiments relate to a neutron and gamma-ray based detection system and method that is cost effective, and fabricated from readily available materials using an organo and organo-metal scintillating material and Gd. Specific embodiments are used for detecting radiological and nuclear threats where a single material composition is used for both the neutron and gamma ray detection in the detector system. In a specific embodiment, the system can be used as a passive detector to detect RDD and light to moderately shielded SNM, and the system can also be used, or can alternatively be used, with active interrogation to detect RDD and to detect lightly, moderately, and heavily shielded nuclear material.

Embodiments of the subject detector can be configured to have a high sensitivity for both neutron and gamma detection, with a sufficient discrimination of the neutron and gamma signatures. An embodiment of the subject system allows for high threat detection sensitivity with low false, or nuisance, alarms, and, thus, allows a high commercial throughput of containers, trucks, and rail traffic.

Embodiments are directed to a neutron and gamma detection system and method that are cost-effective, large area, and wherein a single material forming the detector is fabricated from readily available materials that are individually available from multiple suppliers at low cost.

Importantly, the scintillating material is contained in thin-walled aluminum container units. This protection from the environment ensures that the detector operation is not affected by a high and variable humidity, which has been shown to be a severe problem for the bare plastic scintillator used in spectroscopic RPM (S-RPM) technology.

The present specification discloses multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice embodiments of the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention.

Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.

A detector incorporating a 3"x 3" x 3" cup was filled with the organo-metal material of Table 2 and optically coupled with a PMT. This detector was exposed to a Cs source of 662 keV gamma rays, which were collimated to strike the center of the front face of the detector. The measured spectrum of energy loss in the detector is shown in Fig. 1.

The deconvolved spectrum is also shown in Fig. 1 and has a resolution of 4.3% FWFDVI at 662 keV. This deconvolution process combined the information from the Compton edge and the photopeak with Monte Carlo simulated detector response functions. This result can be compared with the Spectroscopic Plastic Scintillator (SPS) result of 6% FWHM for the 1 meter (M) "plank" (91cm x 25cm). That result is based on a Compton edge, but no photopeak information. The fraction of Photo-peak to total events in the 3" cup was measured to be 10.5%. This was consistent with a MC simulation of the detector response.

The same MC simulation was used to simulate the response of an embodiment 6" x 6" x

12" detector at 0.5, 1.0, 2.0 and 2.5 MeV. The predicted Compton Interaction Probability and Photo-Peak to Total ratios of the responses of the detector are shown in Table 1. In addition, Table 1 contains available data from the Spectroscopic Plastic Scintillator (SPS). Finally, the Table shows the deconvolved energy resolution of our data, and also, the data from SPS.

In a passive mode of radiation detection, embodiments of the subject invention detect gamma rays in the energy range of 0.1 MeV to 3.0 MeV by methods well known in the art. However, the reliable detection of nuclear material may necessitate the use of active interrogation. A new radiation signature unique to nuclear material (including plutonium and enriched uranium) was identified in 2003 at the University of California Radiation Laboratory, which utilized the continuous spectrum of high-energy gamma rays (3-7 MeV) resulting from the decay of isotopes produced from fission, (UCRLID-155315). These fission products are produced as a result of active interrogation. Fortunately, this high-energy gamma-ray signature is robust in that it is very distinct compared to background radiation from all normal isotopes, where the gamma energies of all normal isotopes are all less than 3 MeV. Importantly, a measurable fraction of high energy gamma rays produced by the fission products can penetrate two meters of typical cargo, having an average density of about 0.5 gm/cm³, between nuclear materials at the center of a container to the side wall. Consequently, the signature gamma ray flux at the container wall is measurable with a reasonably large area detector. This flux of high energy gamma rays facilitates the detection of nuclear material, even when the nuclear material is "heavily shielded".

In the last 10 years, both neutron and gamma ray active interrogation beams have been used to demonstrate the flux of delayed high energy gamma rays emitted by SNM. A good review of "Active Interrogation Methods for Detection of Special Nuclear Material" was given in a PhD dissertation by H. Yang at the University of Michigan (2009). Fortunately, the fission product gamma rays have a lifetime of about 20-30 seconds. This lifetime is well matched to acceptable scanning speeds for a continuously moving container. This explains the motivation for the detector material to reliably detect up to 7 MeV gamma rays with high intrinsic detection efficiency and adequate energy resolution to minimize backgrounds.

The spectrum of high energy gamma rays produced by the fission products in active interrogation is essentially continuous due to the very large number of actinide isotopes produced by the active interrogation beam. Without the presence of unique gamma ray energies, which produce peaks in the spectrum, there is no need for the gamma detector to have a high resolution in this energy range (3-7 MeV). The modest energy resolution of embodiments of the subject detector, namely FWHM of 5 to 7% in the range 3-7 MeV is adequate to determine the unique signal indicating the presence of nuclear material. The decay time constant for these high energy gamma rays is about 20 seconds.

Example:

In an embodiment, a scintillating material composition described in Table 2 was used in a detector to detect neutrons and gamma rays. TABLE 2A: COMPOSITION OF AN ORGANO AND ORGANO-METALLIC SCINTILLATING MATERIAL

1) 33% wt. /wt. Diisopropyl naphthalene (DIN)

2) 20% wt. /wt. polyvinyl toluene (molecular weight; 100,000) (PVT)

3) 45% wt. /wt. tetra n-butyl tin, (TnBSn)

4) 2% wt. /wt. 2, 5-diphenyloxazole (PPO)

5) 0.01% wt. /wt. p-bis (2-methylstyryl) benzene (bis-MSB)

TABLE 2B: COMPOSITION OF AN ORGANO AND ORGANO-METALLIC SCINTILLATING MATERIAL

1) 69% wt. /wt. Diisopropyl naphthalene (DIN)

2) 30% wt. /wt. tetra n-butyl tin, (TnBSn)

3) 1% wt. /wt. 2, 5-diphenyloxazole (PPO)

4) 0.005% wt. /wt. p-bis (2-methylstyryl) benzene (bis-MSB)

TABLE 3A: COMPOSITION OF AN ORGANO AND ORGANO-METALLIC SCINTILLATING MATERIAL

1) 48% wt. /wt. Diisopropyl naphthalene (DIN)

2) 5% wt. /wt. polyvinyl toluene (molecular weight; 100,000) (PVT)

3) 45% wt. /wt. tetra n-butyl tin, (TnBSn)

4) 2% wt. /wt. 2, 5-diphenyloxazole (PPO)

5) 0.01% wt. /wt. p-bis (2-methylstyryl) benzene (bis-MSB)

TABLE 3B: COMPOSITION OF AN ORGANO AND ORGANO-METALLIC SCINTILLATING MATERIAL FORMING A NON-POURABLE GEL

1) 68.5% wt. /wt. Diisopropyl naphthalene (DIN)

2) 30% wt. /wt. tetra n-butyl tin, (TnBSn)

3) 1% wt. /wt. 2, 5-diphenyloxazole (PPO)

4) 0.005% wt. /wt. p-bis (2-methylstyryl) benzene (bis-MSB)

5) 0.5%) wt. /wt. of PPO-400 [amine terminated poly(propylene oxide) (Jeffamine D-400) Embodiments of the scintillator composition in accordance with the subject invention comprise:

33-69% wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

Such embodiments can further comprise:

0.3-1%) wt. of PPO-400 [amine terminated poly(propylene oxide).

Embodiments of the scintillator composition in accordance with the subject invention comprise:

33-69%) wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn);

1-1.3% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.007% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

33-68.9% wt. Diisopropyl naphthalene (DIN);

0.1-36% wt. /wt. polyvinyl toluene (molecular weight; 100,000) (PVT),

wherein the % wt of Diisopropyl naphthalene (DIN) and polyvinyl toluene (molecular weight; 100,000) (PVT) totals 33-69% wt.;

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

33-69%) wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn); 1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB),

wherein the %> wt of Diisopropyl naphthalene (DIN), tetra n-butyl tin, (TnBSn), 2, 5- diphenyloxazole (PPO), and p-bis (2-methylstyryl) benzene (bis-MSB) totals at least 99.9% wt.

33-68.9% wt. Diisopropyl naphthalene (DIN);

0.1-36% wt. /wt. polyvinyl toluene (molecular weight; 100,000) (PVT),

wherein the %> wt of Diisopropyl naphthalene (DIN) and polyvinyl toluene (molecular weight; 100,000) (PVT) totals 33-69% wt.;

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB),

wherein the %> wt of Diisopropyl naphthalene (DIN), polyvinyl toluene (molecular weight; 100,000) (PVT), tetra n-butyl tin, (TnBSn), 2, 5-diphenyloxazole (PPO), and p-bis (2- methylstyryl) benzene (bis-MSB) totals at least 99.9% wt.

33-69%) wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO);

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB); and

0.3-1%) wt. of PPO-400 [amine terminated poly(propylene oxide),

wherein the %> wt of Diisopropyl naphthalene (DIN), tetra n-butyl tin, (TnBSn), 2, 5- diphenyloxazole (PPO), and p-bis (2-methylstyryl) benzene (bis-MSB), and PPO-400 [amine terminated poly(propylene oxide) totals at least 99.9%> wt. A gelling agent, such as physical cross-linking agent (for example, PPO-400 [amine terminated poly(propylene oxide) (Jeffamine D-400]), which creates hydrogen bonding, can be used with scintillator compositions in accordance with embodiments of the subject invention, such as the scintillator compositions of Tables 2A, 2B, and 3 A (and 3B as described), where the % wt of one or more of the other components is reduced by the % wt of the gelling agent included. In specific embodiments, 0.3%-l% wt., 0.3%-0.9% wt., 0.3%-0.8% wt., 0.3%-0.7% wt., 0.4%-0.6% wt., 0.45%-0.55% wt., and/or 0.49%-0.51% of the physical cross-linking agent (for example, PPO-400 [amine terminated poly(propylene oxide) (Jeffamine D-400]), which creates hydrogen bonding, can be incorporated into the subject scintillator composition. This technique for incorporating the physical cross-linking agent (for example, PPO-400 [amine terminated poly(propylene oxide) (Jeffamine D-400]), which creates hydrogen bonding, and data regarding PPO-400 [amine terminated poly(propylene oxide) (Jeffamine D-400], is described in D. Savin et al. "Soft Matter" (2016, 12, 4991-5001), which is incorporated by reference into the subject application in its entirety. Regarding the material described in Table 2A, the material viscosity is about 10 centipoise, which is equivalent to slightly viscous water. A small increase in PVT concentration, e.g., a 2% wt. / wt. increase in PVT concentration, coupled with a corresponding reduction of DIN or TnBSn, e.g., a 2% wt. / wt. reduction of DIN or TnBSn, results in an increase in viscosity by a factor of 5.

Regarding the material described in Table 2B, the material viscosity is about 1 centipoise, which is equivalent to water. Other factors to take into consideration with respect to a scintillating material, including auto-ignition temperature, toxicity, flammability, and biodegradability, have been studied and are such that the material described is adequately safe for deployment.

There is an American National Standards Institute (ANSI) requirement that the scintillating material operate successfully at -30C. This requires that the scintillating material maintains excellent optical transmission at that temperature. An embodiment can use the material described in Table 3B, in order to meet this temperature based requirement.

Further embodiments can use a scintillating material described in Table 3A(or a scintillating material having components 3, 4, and 5 as described in Table 2A and Table 3 A, with 48%-33% of component 1, and 5%-20% of component 2) in place of the material described in Table 2A, with respect to embodiment as described herein incorporating the material described in Table 2A. For this reason, the PVT concentration is preferably less than or equal to 7% wt./wt. depending on the molecular weight of the polymer.

Embodiments can use a scintillating material with a substantially higher viscosity than the material of Table 2 A, or the material of Table 2B. Use of a stearate compound in the scintillating material, such as aluminum stearate at a concentration in the range 0.1 to 0.3 wt./ wt. increases the viscosity and still maintains excellent optical transmission for the scintillating material. Other stearate compounds, such as dibuty distearate tin can also be used in the scintillating material. Other compounds such as bismuth neodecanoate may also be used in the scintillating material. The concentration of such compounds are limited by requiring that the compounds remain in solution at the lowest planned operating temperature.

Finally, a gel can be formed from a cross-linkable form of PVT copolymer with divinylbenzene, such that cross-linkable form of PVT copolymer with divinylbenzene can be used as a gelling agent as described above. The polymer can be used at low concentration (<5% wt./wt.) and with a small amount (0.01% - 0.1%) of cross-linking agent together with an initiator, via methods well known in the field of polymer chemistry, see for example, Dow document: DVB: Cross-link a variety of materials for improved thermal, physical, and chemical properties accessed on 03/09/16.

(http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0953/0901b803809

530f8.pdf?filepath=specialtymonomers/pdfs/noreg/503-00002.pdf&fromPage=GetDoc) which is hereby incorporated by reference. In a specific embodiment, a scintillating material is used that has high viscosity like molasses, such that it is difficult to pour, and in another embodiment, a scintillating material can also be in the form of a gel which cannot be poured.

In various embodiments, the relative concentrations of components of the material of Table 2A, or the material of Table 2B can be significantly changed. For example, the concentrations of one or more components are varied by up to + or - 20 %, such that reasonable functionality is still provided by the modified material composition.

In various embodiments, one or more chemical compounds of the material of Table 2A are replaced by another chemical compound that has similar chemical properties. Examples include: 1) Instead of DIN, an aromatic solvent such as xylene or pseudocumene, or a combination of DIN, an aromatic solvent, xylene, and/or pseudocumene, where the % wt. of the replacement or combination is the same as given for DIN, is used, which results in greater toxicity and flammability. Preferably linear alkyl benzene (LAB) with formula C₆H₅CnH₂n ₊i (n=10-13 } can be used, which has a high flash point of 130C. This compound can have excellent optical transmission.

2) Instead of PVT, another polymer, such as polystyrene or polyvinylcarbazole, or a combination of PVT, another polymer, polystyrene, and/or polyvinylcarbazole, where the % wt. of the replacement or combination is the same as given above for PVT, is used. 3) Instead of TnBSn, Tetramethyltin (TMSn) and/or one or more other organometallic compounds (which may include tin and other metals, such as bismuth), or a combination of TnBSn, Tetramethyltin (TMSn), and/or one or more other organometallic compounds (which may include tin and other metals, such as bismuth), where the % wt. of the replacement or combination is the same as given above for TnBSn, may be used. Replacing the TnBSn with TMSn permits the scintillating material composition to contain up to 30% wt./wt. of tin. However, TMSn is more toxic than TnBSn.

4) Instead of PPO, another primary dye, such as paraterphenyl (PTP), any alkyl substituted compound of PPO and PTP, or a combination of PPO, another primary dye, paraterphenyl (PTP), and/or any alkyl substituted compound of PPO and PTP, where the % wt. of the replacement or combination is the same as given above for may be used.

5) Instead of bis-MSB, another secondary dye, such as POPOP, may be used, and any alkyl substituted compound of bis-MSB and POPOP, and a combination of bis-MSB, another secondary dye, POPOP, and/or any alkyl substituted compound of bis-MSB and POPOP, where the % wt. of the replacement or combination is the same as given above for bis- MSB, may be used.

In an embodiment, a specific material functionality of the scintillating material, such as a desired tin content, may be achieved with a combination of two or more compounds, such as a combination of TnBSn + TMSn. In an embodiment, 0.05 to 0.2 % wt./wt. of gadolinium (III) 2-ethylhexanoate is dissolved in the scintillating material composition coupled with a corresponding reduction of one or more of the other components. This scintillating material provides neutron detection by delayed detection of the gamma-rays produced by the Gd after capture of the thermal neutron by the Gadolinium.

Various combinations of compounds can be selected based on one or more factors and/or material properties, including but are not limited to, solubility, light output, known long-term stability, availability in adequate purity from multiple vendors, low cost, providing good Pulse Shape Discrimination, providing good peak to total ratio for gamma ray detection and a desired viscosity of the material.

The energy resolution of an embodiment of a detector containing the scintillating material of Table 2 A was measured at 662 keV using a Cs-137 source. This measurement was made with a cup whose dimensions were 3" x 3" x 3", and the measured spectrum of scintillation pulse heights from gamma rays is shown in FIG. L where a photo peak is clearly seen. The measured FWHM from FIG. l is about 21%. For a comparison, plastic or liquid scintillators do not show any photopeak. The existence of the photopeak in FIG. 1 is due to the presence of tin in the scintillating material of Table 2. The size of the cup is such that there is a low probability to contain all the energy of the gamma ray in the limited size of the cup. In addition, there was little collimation of the gamma ray beam directed at the cell such that many gamma rays struck the cup near the edge of the detector and lost only a small amount of energy in the scintillating material. The measured pulse height spectrum is consistent with a Monte Carlo (MC) simulation of the process.

A Monte Carlo calculation was performed to study the gamma ray fractional energy containment (peak to total ratio) versus thickness of a detector incorporating the scintillating material of Table 2. The result of that Monte Carlo calculation for 5 MeV gamma rays is shown in FIG.2, which shows the average deposited energy fraction of an incident 5 MeV energy gamma ray in an 8" x 8" cross-section detector as a function of detector thickness.

It is seen that an 8" deep detector incorporating the scintillating material of Table 1 will be adequate to contain 90% of the total 5 MeV energy of an incident gamma ray. The measured energy resolution at 5 MeV is expected to be about 7% FWHM, in the range of 5-10%, and dominated by quantum statistics, fluctuations in longitudinal energy loss, and light loss differences from different parts of the detector material.

In one embodiment of the detector system, the unit cell of the gamma ray detector has area dimensions of 8" x 8" and a length of 8" where a schematic of the unit cell detector is shown in FIG.3. The interior wall of the aluminum housing is painted with a highly reflective Titanium dioxide (Ti0₂) cross-linked paint. The scintillating material composition described in Table 2 fills the unit cell detector and has the scintillation light read out from one end (8" x 8"), through a Pyrex window. Further embodiments of the unit cell having different dimensions can be utilized, where Wi, W₂ are transverse widths and D is the depth of the detector, where Wi, W₂ are 1" < W_i;2 < 15" and 5" < D < 12". One or more photo multiplier tubes (PMT) can be used for readout from a given unit cell.

In another embodiment, shown in FIG. 4, a lm x lm area gamma ray detector has 25 (as a 5x5 array) unit cell detectors as shown in FIG. 3.

In another embodiment, a unit cell of a detector that performs neutron detection as well as gamma ray detection, is shown schematically in FIG.5. The organo and organo-metal scintillating material, such as disclosed in Table 2, is contained in two metal containers between which a thin film containing gadolinium is positioned. A 0.1 cm thick steel foil is painted with a specular reflective paint. The foil is cut into the sizes required for the sides of the container. The edges of the foil forming the sides are sealed together by welding. An alternative method of forming the metal box is to use commercially available extruded aluminum with dimensions of 8" x 4" x 63" as shown in Fig. 6. Other cross sectional dimensions are also commercially available at low cost. At one end of the container, a fixture is welded in place to seal the container, where the fixture accommodates a glass plate to form a pressure seal to contain the organo-metallic scintillating material. A 2", 3" or 5" PMT with enhanced photo quantum yield is mechanically held under pressure to form a seal using optical grease between the PMT and the glass plate. This embodiment provides units for SS-RPM and, in addition, a shorter version of the unit cell which can be used as a "backpack" detector of gamma rays and neutrons.

Gadolinium based paint is prepared using gadolinium (111) oxide. The very stable, white powder has a melting point of 2310°C and density of 7.4. The powder is available at 99.99% purity at a price of $0.4/g. The compound is non-hygroscopic and non-air sensitive. The material is mixed with epoxy and painted onto a 0.005" thick Mylar sheet, or aluminum sheet, and a second identical sheet is pressed on the opposite side. The gadolinium plate is kept compressed for the duration of curing. The area of the gadolinium plate may be formed such that it is less than the area of the two detector scintillating components. The outer edge of the gadolinium plate may be located 1" inside the outer edge of the gamma ray detecting material in the containers to ensure efficient detection of most, if not all, of the captured gamma rays produced by the capture of a thermal neutron. These gamma-rays are emitted isotropically from the gadolinium where the thermal neutron was captured. As a result, the gamma rays have a high probability of being detected and releasing a total energy of 4 to 8 MeV in the two adjacent container units of a unit cell.

In another embodiment, the cell units described in the embodiment in FIG. 5 can be made about 60" in length. In this case, the inside of the metal container is painted or treated with a highly reflective material, such as chrome plating. Two containers having scintillating material within them and having a gadolinium layer of material positioned between them form a unit cell. Four such cell units can be contained within a standard National Electrical Manufacturers Association (NEMA) RPM enclosure as shown in FIG. 6. This type of enclosure is used in all existing RPMs. In many cases one or two such enclosures are used on each side of the path of a container being transported. The geometric area of the detector shown in FIG. 6, having the four cell units of FIG. 5, is about 1950 square inches and provides an effective sensitive area for gamma rays and neutrons of about one square meter.

The average intrinsic detection efficiency of 0.5 - 3 MeV gamma rays and fast neutrons in that area are calculated to be about 0.6 and 0.3 respectively. The product of sensitive area (1 square meter) and intrinsic detection efficiencies (fraction) for gamma rays and neutrons are 0.6 and 0.32 respectively. In addition, there are a total of four enclosures per RPM which results in total "sensitivities" of gammas and neutrons of 2.4 and 1.28 respectively. A comparable analysis of Symetrica portals gives corresponding "sensitivities" of 5 to 10 less than those of the subject SS-RPM. This difference in sensitivity for passive detection of RDD and SNM is a powerful demonstration of the advantage of a single material to detect both forms of radiation and improved intrinsic detection efficiency of the scintillating material composition of Table 2. In embodiments using long length detector units, such as 60", readout by photo-sensitive detectors at each end provides the relative pulse heights at each end of the detector. Although the total pulse height produced by a gamma ray interaction can be obtained, it is difficult to obtain the necessary good energy resolution provided by units illustrated in Fig. 4. However, the latter units require more photo readout devices for a given detector area.

In a preferred embodiment, the cell units have dimensions of 6" x 6" and are 12" in depth. This type of unit is shown in Figure 7A and an array of such units is shown in Figure 7B, where the Gd is painted on the outside surface of the middle section of the cell unit. The Gd is located in the mid region of the cell unit so that gamma rays given off by Gd that captured thermal neutron can be measured is goes forward or back ward. In figure 10, and other embodiments, the Gd can be limited to the surfaces on cell units that are inside from the outer edge of the array by the one cell unit.

With respect to all embodiments of a detector for detecting neutrons and gamma rays in accordance with the subject invention, it is preferable to have good discrimination of gamma rays from neutrons. In particular, the passive detection of nuclear material, such as Plutonium and HEU, can best be addressed by neutron detection.

In an embodiment, the scintillating material composition of Table 2, along with Gd, the detector detects fast neutrons and measures the energy of the fast neutrons. Good discrimination of fast neutron events from gamma ray events is also achieved. One or more of four separate methods can be used, each of which independently contribute to achieving a high, or significant, gamma discrimination factor, and when all four are used can achieve a gamma discrimination factor of >10⁶: 1. These four methods are as follows:

1. PULSE SHAPE DISCRIMINATION. A fast neutron incident on the scintillating material of the detector scatters from protons in the scintillating material. The recoil protons produce scintillation pulses with a characteristic prompt time component and a slow, or delayed, time component. The ratio of the magnitude of the delayed time component and the magnitude of the prompt time component is controlled principally by the concentration of the primary dye (PPO) component of the gel or low viscosity scintillating material composition. The purpose of the primary dye is two-fold. First, the primary dye provides enough prompt time component (singlet de-excitation) light for satisfactory operation and energy resolution, and second, the primary dye creates enough delayed time component light from heavily ionizing particles to provide Pulse Shape Discrimination (PSD).

A controllable delayed time constant for the delayed time component is desirable to achieve an optimum separation of the prompt time component and the delayed time component of the scintillation light. This facilitates achieving an acceptable level of PSD, and also tends to maximize the speed of the scintillation counter. For Active Interrogation, where the backgrounds are high, the speed of the counter is an important consideration.

The time structure of each scintillation pulse is measured in order to distinguish whether the event arises from a neutron interaction or an electromagnetic event (gamma ray). Commercial electronics can be used to perform this processing, and, preferably, perform this processing on line.

FIG. 8 shows typical data, using the 3" x 3" x 3" cup of material described in Table 2, allowing pulse shape discrimination between fast neutrons and gamma rays. The ratio of scintillation intensity in the interval 50 to 80 nanoseconds (Tail Integral) is plotted versus the total integral integrated light intensity during the time interval 0 to 80 ns.

A typical PSD discrimination factor of 100: 1, in the range or 100: 1 to 30: 1, and/or 30: 1, against gamma rays appearing like neutrons can be achieved for incident neutron energies greater than about 150 MeV. This discrimination factor is much less than achieved with materials not containing metal. The metal has the effect of quenching the long-lived triplet excited states of the PPO. For this reason, the alkyl structure of the metal compound is important to provide shielding of the PPO from the tin.

2. DELAYED GATE REQUIREMENT

The scintillation pulse produced by one or more scatters of the fast neutron from protons opens an electronic gate about 1 microsecond later, and a measurement is made of the pulse height of events occurring within that gate, having a gate width in the range of 5 to 25 microseconds. During these few microseconds, the fast neutron has been thermalized and has a high probability of being captured by a Gadolinium nucleus. The probability of a random background event occurring in the few microsecond wide gate is dependent on the background counting rate. The background rate is reduced by demanding that the pulse height be greater than 1 to 2 MeV within the delayed gate. The rate of such random events provide a discrimination factor of >100: 1 against incident gamma rays.

3. THE UNIQUE SIGNATURE FROM THERMAL NEUTRON CAPTURE IN THE DELAYED GATE

Upon capture of a thermal neutron by Gadolinium, several low energy gamma rays are released, such that the total energy of the electromagnetic pulse (i.e., the sum of several low energy gamma rays) is 8 MeV. The electromagnetic pulse, which can be summed over the detector units, is measured with a resolution of less than 10%, in the range 5% to 10%, and/or less than 5%. Furthermore, in an embodiment, a coincidence of 2 or more, 3 or more, 4 or more, 5 or more, 6 or more, detector units are required to have greater than 0.5 MeV each, greater than 0.6 MeV each, greater than 0.7 MeV each, greater than 0.8 MeV each, greater than 0.9 MeV each, 1 MeV, and/or greater than 2 MeV energy loss. The use of tin in the scintillating material composition of Table 2 results in a relatively short mean free path for the material, and, in particular, a short mean free path for the several low energy gamma rays in the electromagnetic pulse. The discrimination of the capture of a thermal neutron from events due to background gamma rays is >300: 1, as there are no gamma rays from normal radioactive isotopes with energies above 3 MeV, above 4 MeV, above 5 MeV, above 6 MeV, and/or above 7 MeV. The measured pulse height distribution in an 8" x 8" x 8" container of the material is shown in Figure 3. The logarithmic decrease in the incident gamma ray spectrum is seen and a few neutron events at high energy are seen only when Cf-252 is present.

4. PULSE SHAPE DISCRIMINATION OF THE DELAYED GATE PULSE

The delayed gate pulse distribution above a threshold of 3 MeV is analyzed via PSD as described in method 1 above. Events in the 8 MeV peak (or 3-8 MeV range) of that spectrum, which are selected by PSD to be electromagnetic (i.e., gamma rays), help confirm that a thermal neutron was captured by the Gadolinium. The discrimination against the detected delayed gate event having been caused by a background gamma ray or other event such as a nuclear cosmic event), and not the capture of a thermal neutron by the Gadolinium is greater than 100: 1, in the range 30: 1 to 100: 1, and/or greater than 30: 1. In one embodiment of the invention, the scintillating material of Table 2 and the use of such scintillating material in a Radiation Portal Monitor provides detection of a fast neutron and measurement of the energy of the fast neutron. Identification of the event being produced by a fast neutron rather than a gamma ray is made on the basis of four factors, each providing a discrimination greater than 100: 1, in the range 30: 1 to 100: 1, and/or greater than 30: 1. The four factors are: (i) PSD analysis of the initial pulse;100: l; (ii) requirement of a > lMeV pulse in a delayed electronic gate; 100: 1; and observation of an 8 MeV pulse (or 3-8 MeV pulse) occurring during the time of the delayed gate; >300: 1; and PSD analysis of the 8 MeV delayed pulse (or 3- 8 MeV delayed pulse); 100: 1.

In a preferred embodiment, the gamma ray discrimination is greater than 10⁶: 1; and, in a more preferred embodiment, is greater than 10⁷: 1; and, in an even more preferred embodiment, is greater than 10⁸: 1.

The embodiment of the SS-RPM shown in FIG. 4 or 5 or 10, with aluminum containers instead of stainless steel, when used with Active Interrogation, can detect and measure the energy of very high energy (4MeV to 7 MeV) gamma rays. Delayed gamma-rays produced from the decay of actinide fission products of SNM that are produced by Active Interrogation are typically in the range 0.5 to 7 MeV. With Active Interrogation, the high energy gamma-rays (4 - 7 MeV) produced from the decay of the actinide fission products of SNM are more likely to penetrate the special shielding that might be used to attempt to conceal the signature of SNM, pass through the shielding material, exit the container, and reach the SS-RPM. There are no background sources of gamma-rays at this energy (4 - 7 MeV) and, therefore, Active Interrogation is thought to be the most effective way of detecting shielded SNM. In this way, this embodiment of an SS-RPM can enhance detection of RDD and Shielded SNM when the SS- RPM is opened in passive mode and/or active mode. Thus, delayed high energy gamma rays and neutrons will be measured as signatures of SNM.

This embodiment of an SS-RPM, using a single scintillating material, will meet and exceed all necessary International standards. The scintillating material has a controllable viscosity and can be modified to be a weak gel with little or no, ability, to be poured. The scintillation intensity of this scintillation material has been measured to be constant to within a 5% range in the temperature range -30C to 45C. This scintillation material has a boiling point greater than 130C and low toxicity. The cost of this scintillation material is expected to be about twice that of standard liquid scintillation materials, and a few percent that of Sodium Iodide.

Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention.

Aspects of the invention may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with a variety of computer- system configurations, including multiprocessor systems, microprocessor-based or programmable-consumer electronics, minicomputers, mainframe computers, and the like. Any number of computer- systems and computer networks are acceptable for use with the present invention.

As one skilled in the art will appreciate, embodiments of the present invention may be embodied as, among other things: a method, system, or computer-program product. Accordingly, the embodiments may take the form of a hardware embodiment, a software embodiment, or an embodiment combining software and hardware. In an embodiment, the present invention takes the form of a computer-program product that includes computer-useable instructions embodied on one or more computer-readable media.

Communication between network elements may be wireless or wireline (wired). As will be appreciated by those skilled in the art, communication networks may take several different forms and may use several different communication protocols. And the present invention is not limited by the forms and communication protocols described herein.

The examples and embodiments described herein are for illustrative purposes only and various modifications or changes in light thereof will be apparent to persons skilled in the art and are included within the spirit and purview of this application. In addition, any elements or limitations of any invention or embodiment thereof disclosed herein can be combined with any and/or all other elements or limitations (individually or in any combination) or any other invention or embodiment thereof disclosed herein, and all such combinations are contemplated with the scope of the invention without limitation thereto. All patents, patent applications, provisional applications, and publications referred to or cited herein (including those in the "References" section) are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.

REFERENCES

1. BURT, Christopher et al. "The development of large-area plastic gamma-ray spectrometers," Nuclear Science Symposium Conference Record, IEEE, 2008, pages 1186-1190.

2. S ANNIE, G. et al., "Scintilla: A new international platform for the development, evaluation and benchmarking of technologies to detect radioactive and nuclear material," 3^rd International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), June 2013, pages 1-4.

3. U.S. Patent No. 5,006,299

4. U.S. Patent No. 5,076,993

5. U.S. Patent No. 5,114,662

6. U.S. Patent No. 8,963,094

7. U.S. Serial No. 13/430,394

8. YANG, Haori. "Active Interrogation Methods for Detection of Special Nuclear Material," University of Michigan dissertation, 2009, pages 1-216.

Claims

CLAIMS What is claimed is:

1. A scintillator composition, comprising:

33-69% wt. of:

Diisopropyl naphthalene (DIN);

xylene;

pseudocumene; and/or

linear alkyl benzene (LAB);

29-66% wt. of:

tetra n-butyl tin, (TnBSn); and/or

Tetramethyltin (TMSn);

1-2% wt. of:

2, 5-diphenyloxazole (PPO);

paraterphenyl (PTP), and/or

any alkyl substituted compound of PPO and PTP; and 0.005-0.01% wt. of:

p-bis (2-methylstyryl) benzene (bis-MSB); POPOP, may be used, and/or

any alkyl substituted compound of bis-MSB and POPOP

2. A scintillator composition, comprising:

33-69%) wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

3. The scintillator composition according to claim 2, comprising:

33-69%) wt. Diisopropyl naphthalene (DIN);

29-66% wt. tetra n-butyl tin, (TnBSn); 1-1.3% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.007% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

4. The scintillator composition according to claim 2, comprising:

33-68.9% wt. Diisopropyl naphthalene (DIN);

0.1-36% wt. of:

polyvinyl toluene (molecular weight; 100,000) (PVT);

polystyrene; and/or

polyvinylcarbazole,

wherein the %> wt of Diisopropyl naphthalene (DIN), polyvinyl toluene (molecular weight; 100,000) (PVT), polystyrene; and/or polyvinylcarbazole totals 33-69%) wt.;

29-66% wt. tetra n-butyl tin, (TnBSn);

1-2% wt. 2, 5-diphenyloxazole (PPO); and

0.005-0.01% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

5. The scintillator composition according to claim 2, further comprising:

0.3-1%) wt. of PPO-400 [amine terminated poly(propylene oxide).

6. The scintillator composition according to claim 2, further comprising:

0.3-1% wt. of a cross-linkable form of PVT copolymer with divinylbenzene.

7. The scintillator composition according to claim 2, further comprising:

0.3-1% wt. of:

PPO-400 [amine terminated poly(propylene oxide); and/or

a cross-linkable form of PVT copolymer with divinylbenzene.

8. The scintillator composition according to claim 2,

9. The scintillator composition according to claim 4,

wherein the % wt of Diisopropyl naphthalene (DIN), polyvinyl toluene (molecular weight; 100,000) (PVT), tetra n-butyl tin, (TnBSn), 2, 5-diphenyloxazole (PPO), and p-bis (2- methylstyryl) benzene (bis-MSB) totals at least 99.9% wt.

10. The scintillator composition according to claim 5,

wherein the % wt of Diisopropyl naphthalene (DIN), tetra n-butyl tin, (TnBSn), 2, 5- diphenyloxazole (PPO), and p-bis (2-methylstyryl) benzene (bis-MSB), and PPO-400 [amine terminated poly(propylene oxide) totals at least 99.9% wt.

1 1. The scintillator composition according to claim 2, comprising:

69% wt. Diisopropyl naphthalene (DIN);

30% wt. tetra n-butyl tin, (TnBSn);

1% wt. 2, 5-diphenyloxazole (PPO); and

0.005% wt. p-bis (2-methylstyryl) benzene (bis-MSB).

12. The scintillator composition according to claim 5, comprising:

68.5%) wt. Diisopropyl naphthalene (DIN);

30% wt. tetra n-butyl tin, (TnBSn);

1% wt. 2, 5-diphenyloxazole (PPO);

0.005% wt. p-bis (2-methylstyryl) benzene (bis-MSB); and

0.5%) wt. PPO-400 [amine terminated poly(propylene oxide).

13. A scintillation system for detecting incident radiation, comprising the scintillator composition of any of the preceding claims.

14. The scintillation system according to claim 13, further comprising:

gadolinium, wherein the gadolinium captures thermal neutrons incident on the gadolinium and produces several gammas rays,

wherein the scintillation system comprises a plurality of detector units, wherein each detector unit encloses a portion of the scintillator composition,.

15. The scintillation system according to claim 14,

wherein the gadolinium is a soluable gadolinium salt of carboxcylic acid.

16. The scintillation system according to claim 14,

wherein the several gamma rays produced by the gadolinium have a total energy of 8

MeV.

17. The scintillation system according to claim 16,

wherein the scintillation system detects a total energy of 3-8 MeV due to the several gamma rays produced by the gadolinium.

18. The scintillation system according to claim 14,

wherein upon a gamma ray being incident on the scintillator composition,

scintillation light is produced; and

wherein upon a fast neutron being incident on the scintillator composition, scintillation light is produced,

wherein the gadolinium is positioned with respect to the scintillator composition such that:

(i) upon a fast neutron being incident on the scintillator composition,

(a) the fast neutron is thermalized to become a thermalized thermal neutron;

(b) the thermalized thermal neutron is captured by the gadolinium; and

(c) the gadolinium that captured the thermalized thermal neutron produces several gamma rays that produce gadolinium gamma ray scintillation light; and

(ii) upon an incident thermal neutron being incident on the scintillator composition, (a) the incident thermal neutron is captured by the gadolinium;

(b) the gadolinium that captured the thermalized thermal neutron produces several gamma rays that produce gadolinium gamma ray scintillation light,

wherein the scintillation light allows detection of a fast neutron incident on the scintillator composition discriminated against a gamma ray.

19. The scintillation system according to claim 18,

wherein the scintillation system determines a fast neutron was incident on the scintillator composition with a discrimination factor of at least 1,000,000: 1 against gamma rays.

20. A method for detecting incident radiation from a radiation source, comprising:

positioning a scintillation system of claim 14 in a region of interest; and

determining from the scintillation light whether a fast neutron was incident on the scintillation composition.

21. The method according to claim 20,

determining from the scintillation light whether a fast neutron was incident on the scintillation system comprises:

applying pulse shape discrimination to scintillation light produced during an initial period after threshold met;

if a determination made that fast neutron was incident on scintillation composition,

measuring scintillation light produced during a delayed gate period; applying at least one delayed gate criterion to scintillation light produced during a delayed gate period;

applying a signature criterion to scintillation light produced during the delayed gate period; and

applying pulse shape discrimination to scintillation light produced during the delayed gate period.

22. The method according to claim 20,

if a determination made that fast neutron was not incident on scintillation composition, measuring total energy of scintillation light produced during the initial period.

23. The method according to claim 21,

24. The method according to claim 21,

wherein upon a fast neutron being incident on the scintillating composition, fast neutron scintillation light is produced, wherein the fast neutron scintillation light comprises a fast neutron prompt time component and a fast neutron delayed time component,

wherein applying pulse shape discrimination to scintillation light produced during an initial period after threshold met comprises:

determining a ratio of a magnitude of the fast neutron delayed time component and the magnitude of the fast neutron prompt time component; and

determining a fast neutron was incident on the scintillator composition.

25. The method according to claim 24,

wherein the scintillation system determines a fast neutron was incident on the scintillator composition with a first discrimination factor of at least 30: 1 against gamma rays.

26. The method according to claim 24, wherein upon determining that the fast neutron was incident on the scintillating composition, measuring the scintillation light produced during a delayed gate period, wherein the delayed gate period starts after a delay period after the initial period began;

determining a sum of scintillation light measured during the delayed gate period, wherein applying the at least one delayed gate criterion to scintillation light produced during the delayed gate period comprises:

when the sum of scintillation light measured during the delayed gate period by detector units is greater than 2 MeV, a determination that a fast neutron was incident on the scintillating composition is made; and

when the sum of scintillation light measured during the delayed gate period by detector units is not greater than 2 MeV, a determination that a fast neutron was not incident on the scintillating composition is made.

27. The method according to claim 26,

wherein the delay period is at least 1 microsecond after the initial period started, wherein the delayed gate period is 10 to 30 microseconds.

28. The method according to claim 26,

wherein the determination that a fast neutron was incident on the scintillating composition made based on applying the at least one delayed gate criterion to scintillation light produced during the delayed gate period provides a second discrimination factor of greater than 100: 1 against gamma rays.

29. The method according to claim 26,

wherein the gadolinium gamma ray scintillation light have a total energy of 8 MeV, measuring the scintillation light detected by each detector unit,

summing measured scintillation light of the detector units,

wherein applying signature criterion to scintillation light produced during the delayed gate period comprises: when the sum of the measured scintillation light of the detector units is at least 2 MeV, and

the measured scintillation light of at least two detector units is at least 1 MeV each, a determination is made that:

(i) the thermalized thermal neutron was captured by the gadolinium; or

(ii) the incident thermal neutron was captured by the gadolinium.

30. The method according to claim 29,

wherein the determination is made that:

(i) the thermalized thermal neutron was captured by the gadolinium; or

(ii) the incident thermal neutron was captured by the gadolinium

a third discrimination factor of at least 300: 1 from events due to background gamma rays.

31. The method according to claim 29,

wherein when the determination is made that:

(i) the thermalized thermal neutron was captured by the gadolinium; or

(ii) the incident thermal neutron was captured by the gadolinium, applying pulse shape discrimination to scintillation light produced during the delayed gate period upon a measured pulse during the delayed gate period exceeding a threshold,

wherein pulse shape discrimination is applied during a second initial period after the measured pulse exceeded the threshold,

wherein when a sum of the measured scintillation light of the detector units is at least 3

MeV,

a determination is made that:

(i) the thermalized thermal neutron was captured by the gadolinium; or

(ii) the incident thermal neutron was captured by the gadolinium.

32. The method according to claim 31,

wherein the determination is made that: (i) the thermalized thermal neutron was captured by the gadolinium; or

(ii) the incident thermal neutron was captured by the gadolinium

based on applying pulse shape discrimination to scintillation light produced during the delayed gate period provides a fourth discrimination factor of at least 30: 1 against gamma rays.

33. The method according to claim 32,

wherein the first discrimination factor, the second discrimination factor, the third discrimination factor, and the fourth discrimination factor are independent.

34. The method according to claim 33,

wherein a cumulative discrimination factor is the product of the first discrimination factor, the second discrimination factor, the third discrimination factor, and the fourth discrimination factor are independent.

35. The method according to claim 34,

wherein a cumulative discrimination factor is at least 1,000,000: 1 against gamma rays.

36. The method according to claim 34,

wherein a cumulative discrimination factor is at least 10,000,000: 1 against gamma rays.